Definition of Quantum Yield
Quantum yield (Φ) is a fundamental concept in photochemistry, quantifying the efficiency of a photochemical or photophysical process. It is defined as the number of times a specific event occurs per photon absorbed by the system. This event could be a chemical reaction, fluorescence, phosphorescence, or any other molecular process initiated by light absorption.
Key Principles and Calculation
The quantum yield is typically a value between 0 and 1 (or 0% and 100%). A quantum yield of 1 (or 100%) means that every absorbed photon leads to the desired event. It is calculated by dividing the number of specific events (e.g., molecules reacted, photons emitted) by the number of photons absorbed. Factors like temperature, solvent, and the presence of quenchers can significantly influence quantum yield.
A Practical Example
In fluorescence, the fluorescence quantum yield (Φf) measures the efficiency of photon emission. If a molecule absorbs 100 photons and subsequently emits 30 photons as fluorescence, its fluorescence quantum yield is 0.3 or 30%. This indicates that 30% of the absorbed light energy is re-emitted as fluorescent light, while the remaining 70% is dissipated through other pathways like heat or non-radiative decay.
Importance and Applications
Quantum yield is critical in various scientific and technological fields. In biochemistry, it helps characterize fluorescent probes and understand molecular interactions. In solar energy, it's used to evaluate the efficiency of photovoltaic materials. It's also vital in developing new photocatalysts, light-emitting diodes (LEDs), and photodynamic therapies, as a higher quantum yield often correlates with better performance.